The present invention relates to digital processing techniques for graphics and, more particularly, to a method and means for thresholding image data to reduce the information content for transmission and reproduction.
The general concept of thresholding in connection with image data handling to reduce the information content of an image to essentially black and white form is well known. The many related prior art techniques primarily deal with how the thresholding decisions are made and typical approaches are variously taught, for example, in U.S. Pat. No. 4,251,837 to JANEWAY, IBM Technical Disclosure Bulletin, Vol. 14, No. 4, September 1971, CHOW ET AL, pages 1301-1304, as well as in U.S. Pat. Nos. 4,439,789 to CAHILL, 4,238,768 to MITSUYA ET AL, and 4,326,258 to de la GUARDIA.
The essential problem in thresholding is to identify (1) the intensity range in an image which is to be considered background, (2) the intensity range which is to be considered foreground, that is, the text areas, and (3) the color of the background, that is, whether black or white. Any thresholding technique is preferably simple from a computational point and yet reliable. A convenient method of analyzing intensity ranges in this manner is with the use of an image histogram, that is, a plot of the intensity distribution of the image. One prior art example of thresholding with a histogram is found in the cited IBM Technical Disclosure Bulletin of CHOW ET AL wherein thresholds are calculated by computing the histogram for each region of an image and then determing the thresholds from the estimated distribution by the method of maximum likelihood. The de la GUARDIA patent also discusses the setting of thresholds using the histogram of FIG. 8. Both of these systems, however, have a fair degree of complexity. Other techniques involve histogram peak location and while perhaps less complex frequently lack reliability.
By comparison the present invention discloses a system that is both simple and reliable using only very easily obtained histogram parameters and which, when combined with shading correction techniques, such as disclosed in co-pending U.S. patent application field concurrently herewith, U.S. application Ser. No. 778,632 filed Sept. 20, 1985, is very robust.